Deacidizing gaseous effluents such as, for example, natural gas and combustion fumes, is generally carried out by washing with an absorbent solution. The absorbent solution allows the acid compounds present in the gaseous effluent (H2S, mercaptans, CO2, COS, SO2, CS2) to be absorbed.
Deacidizing these effluents, notably decarbonation and desulfurization, imposes specific requirements on the absorbent solution, in particular thermal and chemical stability, notably towards the impurities in the effluent, i.e. essentially oxygen, SOx and NOx. The oxygen can also come into contact with the absorbent solution without being necessarily present in the gaseous effluent to be treated, for example in case of an accidental air inflow in the absorbent solution storage tanks.
Currently, the most commonly used absorbent solutions are aqueous alkanolamine solutions. Document FR-2,820,430 provides gaseous effluent deacidizing methods.
It is however well known to the person skilled in the art that these amines involve the drawback of degrading under the conditions of use.
In particular, the amines can be degraded by oxygen, generating an amine consumption and the formation of degradation products that accumulate in the unit or, for the most volatile ones, that are carried along in the gaseous effluents of the process. Thus, in particular in the case of post-combustion fumes treatment in a process using an aqueous monoethanolamine (MEA) solution, large amounts of ammonia form. The ammonia thus formed is carried along into the atmosphere with the treated fumes, which poses environmental protection problems.
In the case of CO2 capture in fumes from industrial or electric power plants or, more generally, from energy production plants, the degradation phenomena that affect amine-containing absorbent solutions are increased by the presence of massive amounts of oxygen in the feed to be treated, up to 5 vol. % in general. In the case of fumes from natural gas combined cycles, the volume proportion of oxygen in the fumes can reach 15%.
The degraded solution is characterized by:    a decrease in the absorption of the acid compounds of the feed in relation to a fresh amine solution,    an increase in the density of the absorbent solution, and in the viscosity thereof, which can lead to a performance loss,    the formation of more volatile amines that pollute the treated gas and the acid gas from the regeneration stage: ammonia, methylamine, dimethylamine and trimethyl-amine for example, depending on the nature of the amine used,    an accumulation of degradation products in the absorbent solution, which may require treating the degraded solution,    possible foaming problems due to the degradation products.
The degradation of the absorbent solution thus penalizes the performance and the smooth functioning of the gas deacidizing units.
In order to overcome the degradation problem, if it is not possible to limit or to suppress the presence of oxygen in the absorbent solution, compounds whose purpose is to prevent or to limit amine compound degradation phenomena, notably the degradation generated by oxidation phenomena, are added to the absorbent solution. These compounds are commonly referred to as degradation inhibiting agents or additives. The main modes of action known for degradation inhibiting agents are, depending on the nature thereof, a reaction of reduction and/or capture, trapping and/or stabilization of the radicals formed in the absorbent solution so as to limit or to prevent or to interrupt the degradation reactions, notably chain reactions.
U.S. Pat. No. 5,686,016 mentions additives used for limiting the degradation of absorbent solutions used for deacidizing natural gas, in particular oximes.
U.S. Pat. No. 7,056,482 mentions additives used for limiting the degradation of absorbent solutions used for CO2 capture, notably thiosulfates and sulfites.
In general terms, the present invention provides a family of degradation inhibiting agents that notably allows to reduce the degradation of an absorbent solution used for absorption of acid compounds contained in a gaseous effluent, the absorbent solution comprising amine compounds in aqueous solution.